Current approaches to the removal of organic contaminants in aqueous streams include the use of adsorptive filters, chemical-UV initiated oxidation, ozone treatment and heterogeneous photocatalysis. Heterogeneous photocatalysis employs photopromoted catalytic degradation of organic material in aqueous solutions and organic solvents. Oxygen is often used as the oxidizing agent in combination with a solid catalyst, such as a semi-conductor transition metal oxide in powder form. Heterogeneous photocatalysis involves the continuous illumination of a photoexcitable solid catalyst to degrade reactants adsorbed on the photocatalyst surface (Ollis et al. (1991a) Environ. Sci. Technol. 25:1523). Laboratory studies demonstrate that a wide range of organic compounds decompose into carbon dioxide and mineral acids when semiconductor powders are added to aqueous wastes and illuminated with long wave ultraviolet (UV) photons (Ollis et al. (1991b) in: Photocatalysis: Fundamentals and Applications (Serpone & Pelizzetti, eds.), John Wiley & Sons, New York, pp. 603-637; Pelizzetti et al. (1988) in: Photocatalysis and Environment (Schiavello, ed.), Kluwer Academic Publishers, Dordrecht, pp. 469-497). Unfortunately, low quantum yields limit the possibilities for economical scale up in industrial applications (Ollis et al. (1991a) supra).
The photoefficiency of photosynthesis is known to be decreased due to the saturation phenomena of photo-bleaching of the photosynthetic pigments. Periodic illumination has been studied in the photo-driven process of photosynthesis to increase photoefficiency (Kok (1956) Biochim. Biophys. Acta 21:245; Laws et al. (1983) Biotech. Bioeng. 25:2319). Semi-conductor based photoconversion devices are not expected to "bleach" or saturate because charge separation occurs by electronic conduction in semi-conductors, as opposed to the movement of electrons in the photosynthetic process. The Applicants' work is the first evidence that controlled periodic illumination increases the photoefficiency of semi-conductor photoconversion.
The prior art describes the use of photocatalytic methods for the degradation of contaminating organic materials in aqueous or organic solutions. For example, Lichtin et al. (U.S. Pat. No. 4,861,484), entitled: Catalytic Process For Degradation of Organic Materials in Aqueous and Organic Fluids to Produce Environmentally Compatible Products, describes the use of a variety of solid transition metal catalysts, including titanium dioxide, in conjunction with hydrogen peroxide to photochemically degrade a wide variety of organic materials into carbon dioxide and other environmentally compatible compounds. Lichtin examines reaction products after illumination times of 1 to 6 hours. Lichtin does not disclose or suggest varying dark recovery times or the use of controlled intermittent illumination/dark recovery periods. Al-Ekabi et al. (U.S. Pat. No. 5,126,111), entitled: Fluid Purification, is directed to improving the quantum yield of TiO.sub.2 photocatalytic degradation of organics by inhibiting the electron/hole pair recombination process. This is accomplished by adding specific electron acceptors to the solution mixture to promote the formation of radical species, which enhances the oxidation of the organic compounds and thereby increases the quantum yield of the photocatalytic process. In contrast to the Al-Ekabi patent, the present invention does not require the addition of specific electron acceptors, but uses controlled periodic illumination to generate electron/hole pairs which create oxidizing species which can react during the dark recovery period.
Kitamori et al. (U.S. Pat. No. 4,774,026), entitled: Process and Apparatus for Oxidizing or Reducing Dissolved Substance, in a similar manner to Al-Ekabi, Kitamori describes a method of improving the efficiency of photocatalysis by the addition of a water soluble electron acceptor. Kitamori does not disclose or suggest varying illumination or dark recovery periods or the use of periodic cycles of illumination/darkness. Raupp (1992)(First International Conference on TiO.sub.2 Photocatalytic Purification and Treatment of Water and Air, abstract, pg. 73) suggests that regeneration of deactivated catalysts may be achieved by flowing humid air over the catalyst bed for brief time periods in the absence of UV illumination, or by allowing the catalyst to rest in the dark for longer periods of time without gas flow. Raupp does not suggest the use of controlled periodic illumination of photocatalytic material nor the use of periodic illumination as a method of improving photoefficiency.